The present invention is directed to a beam generating system for electron beam measuring instruments, particularly, a beam generating system for electron beam measuring instruments having an electron accelerating electrode, a control electrode, a directly heated boride cathode and a holding part for the absorption of the retaining forces generated by the clamping mechanism.
Prior art beam generating systems are disclosed, for example, in J. Vac. Sci. Technol. 15 (4), 1978, pages 1554 through 1560 (in particular, see page 1558, FIG. 7) and in German patent DE 31 50 848. The life expectancy of lanthanum hexaboride (LaB.sub.6) crystals used as cathodes in these beam generating systems is limited by their thermically disadvantageous mount. As a consequence of a temperature gradient existing between a tip of the crystal and a shank of the crystal, an increased evaporation rate occurs in the region of the mount in comparison to the tip. The evaporation rate destroys the mechanical stability of the cathode (see, for example, Scanning Electron Microscopy, 1985/4, pages 1327 through 1338, particularly page 1331, FIG. 6a).